Field of the Invention
The present invention relates to an organic light emitting display element, and more particularly, to an organic light emitting display element in which an organic light emitting layer is protected from damage when an upper electrode is formed.
Description of the Related Art
An organic light emitting display (OLED) is a self-light emitting display device that does not need a separate light source like a liquid crystal display (LCD), and, thus, OLED devices can be manufactured to be lightweight and thin. Further, OLED devices are efficient in terms of power consumption since it is driven with a low voltage. Also, OLED devices have excellent color expression capability, a high response speed, a wide viewing angle, and a high contrast ratio (CR). Therefore, OLED devices have received much attention for next-generation displays.
In a display that uses OLED elements, it may be preferable to adopt a top-emission type structure in which light from the organic light emitting display element is emitted to an opposite side of a substrate having switching elements thereon, i.e., the so-called TFT back-plane. Adopting a top-emission type structure is advantageous in terms of aspect ratio compared to adopting a bottom-emission type structure in which the light from the organic light emitting display element is emitted to the TFT back-plane for high resolution. However, if the top-emission type structure is adopted, an upper electrode, from which light is emitted, needs to have a relatively high transmissivity to visible light and also a relatively high electrical conductivity. Thus, there would be a limit in applicable materials and in the thickness of various layers thereof.
In other words, in a top-emission organic light emitting display element display, an upper electrode needs tobe transmissive to light. Thus, the upper electrode needs to provide a relative high degree of transparency, while also serve as an electrode. If the upper electrode is a cathode, a work function of such upper electrode needs to be kept low in order to readily inject electrons. However, a metal with a low work function value and a high electrical conductivity has a very low transmissivity to visible light. Thus, if the metal is formed to have a thickness of 200 Å or more, luminance efficiency of the OLED element would be remarkably decreased.
Furthermore, in recent years, as more displays with higher resolution have been developed, a demand for large-area displays has been increased as well. As an area or size of a display increases, a sheet resistance of an upper electrode common to a front surface of a display panel also increases. As a result, non-uniformity is caused between luminance at the edge of the display and luminance at the center of the display. In order to reduce the sheet resistance, the upper electrode needs to have its thickness increased. However, in view of the foregoing, if the metal with a low work function value and a high electrical conductivity is used as a cathode in the upper electrode, it is necessary to secure transmissivity to light. Thus, there are many difficulties in increasing the thickness of the upper electrode.
Accordingly, in the relevant industry, various technologies have been applied to an organic light emitting display element in order to reduce sheet resistance of an upper electrode while securing the required light transmissivity of the upper electrode. According to one of the technologies, an upper electrode is manufactured by forming a metal having a low work function value into a very thin film in order to pass visible light. Further, an upper electrode is manufactured by forming a substance having a high transmissivity to visible light while having an electrical conductivity on the metal thin film by sputtering in order to reduce sheet resistance.
However, the related art method has at least the following problems.
When a substance with a high transmissivity to visible light while having an electrical conductivity is formed on a metal thin film, a TCO (Transparent Conductive Oxide)-based substance, such as ITO and IZO, is typically used. Such substance can be deposited only by using a sputtering method, which is one type of physical vapor deposition (PVD) method. In the sputtering method, cationized molecules (of a target) strike a deposition surface and they may degenerate or damage an already deposited organic light emitting layer, which causes various problems in the structure and/or function of a light emitting layer.
The damage on the organic light emitting layer caused from the sputtering method can be partially prevented by introducing a metal thin film. However, such metal thin film causes a micro-cavity phenomenon due to light reflective properties of metal. Thus, lights in a specific wavelength range cause constructive interference and other lights in a wavelength range that does not cause constructive interference may cause destructive interference instead. As a result, the intensity of light generated from the organic light emitting layer in a specific wavelength range increases. But a wavelength region in an emission spectrum of light generated from the organic light emitting layer becomes smaller. Eventually the white color coordinates may be distorted and the overall efficiency of white light emission may be decreased.
Thus, the inventors of the present invention have introduced a method of doping a metal dopant into an organic layer rather than using a metal thin film. They invented an organic light emitting display element in which such metal dopant does not have or has only minimal negative influences on light emission of an organic light emitting layer.